DESCRIPTION: Many children both in the United States and around the world grow up bilingual, and as a consequence face substantial challenges in language learning. Contrary to popular belief, only a tiny minority of these bilingual children grow up in a strict one-person/one language environment. Instead, most bilingual children regularly hear two different languages from the same person, within the same conversation, and often within the same sentence (i.e., mixed-language sentences such as Look at the perro!). While considerable progress has been made in uncovering how young bilinguals process single-language input, very little is known about how they process mixed-language input. The proposed international project leverages the natural complexity observed in bilingual children's language environments, and provides a rigorous test of whether language mixing impacts processing and learning. Specific Aim 1 is to investigate whether bilingual infants differentiate between speech from a single language vs. a mix of two languages. Based on two preliminary studies, Specific Aim 2 is to investigate whether language mixing influences the speed of real-time language processing in bilingual toddlers. Specific Aim 3 is to investigate whether the hypothesized processing cost of language mixing has consequences for recognizing and learning words that occur after a language switch. Previous research has shown that bilingual infants can discriminate between individual sounds as well as full sentences in their two languages, and studies with bilingual adults have shown robustly that language mixing disrupts language processing. Given these findings, we predict that bilingual infants will successfully discriminate between single-language and mixed-language speech (Experiments 1a and 1b), and will show a processing cost of language mixing that delays word recognition (Experiments 2a and 2b) and impairs the comprehension of familiar words and the learning of novel words that occur downstream (Experiments 3a and 3b). The proposed experiments will be conducted simultaneously in two bilingual communities: Spanish/English bilingual children will be tested in Chicago, IL, U.S.A., and French/English bilingual children will be tested in Montral, Qubec, Canada. This approach enables an examination of differences between two communities to address how individual-level factors including socioeconomic status and household use of mixed language affect bilingual infants' and toddlers' processing of mixed language. The results will illuminate how bilingual infants and toddlers navigate the complexities of mixed-language input, which presents an important real-world challenge for language development. The outcomes of this research will inform a significant health concern about the consequences of different types of early bilingual experience, and address parents' questions about how to best support dual- language development in bilingual infants and toddlers.